The present invention relates to materials which encourage elimination by adsorption of oxides of nitrogen (NO and NO2, usually termed NOx) present in a gas mixture which may be super-stoichiometric in oxidising compounds, and in particular in oxygen, said materials not being poisoned by the sulphur-containing products encountered in those gases. The invention is applicable to eliminating oxides of nitrogen (NOx) present in the exhaust gases from automotive vehicles, in particular from vehicles functioning with diesel fuel.
The high toxicity of oxides of nitrogen and their role in the formation of acid rain and tropospheric ozone have led to the instigation of strict regulations limiting the discharge of such compounds. In order to satisfy those regulations, it is generally necessary to eliminate at least a portion of such oxides present in exhaust gases from automotive or stationary engines and from turbines.
The elimination of oxides of nitrogen by thermal decomposition or preferably by catalytic if decomposition can be envisaged, but the high temperatures demanded by this reaction are incompatible with those of the exhaust gases. Only catalytic reduction of oxides of nitrogen to nitrogen is possible using the reducing agents which are present, albeit in small quantities, in the exhaust gases (CO, H2, unburned hydrocarbons or where combustion in the engine has been imperfect), and also by injecting a complement to those reducing compounds upstream of the catalyst. Such reducing agents are hydrocarbons, alcohols, ethers or other oxygen-containing compounds, they can also be a liquid or gaseous fuel (under pressure, CNG, or liquefied, LPG) feeding the engine or turbine.
European patent EP-A-0 540 280 describes an apparatus for reducing emissions of oxides of nitrogen in the exhaust gases from internal combustion engines, which comprises a material for adsorbing and desorbing oxides of nitrogen. In that process, the oxides of nitrogen are stored in the form of nitrates when the engine is burning lean, i.e., depleted in hydrocarbons. However the storage capacity of a trap operating using that principle is generally deteriorated by adsorption of sulphur-containing products contained in the exhaust gas which form sulphates which are more stable that the nitrates, poisoning the trap.
Further, following NOx trapping, a step for desorbing the oxides of nitrogen must be carried out followed by their reduction. Devices for treatment by catalysed oxidation of carbon monoxide CO and hydrocarbons HC contained in the exhaust gases are known which, for example, use catalysts for reducing oxides of nitrogen, known as NOx catalysts, which are active for reducing NOx in temperature ranges in the range 200xc2x0 C. to 350xc2x0 C. and which comprise, for example, precious metals on oxide supports such as platinum or palladium deposited on an alumina, titanium oxide or zirconium support, or by perovskites, or in temperature ranges in the range 350xc2x0 C. to 600xc2x0 C. comprising, for example, hydrothermally stable zeolites (for example Cu-ZSM5). A device for treating exhaust gases from a compression ignition engine comprising a catalyst and an oxides of nitrogen adsorbent placed in the exhaust collector has been described, for example, in patents EP-A1-0 540 280 and EP-A1-0 718 478.
Thus a material behaving as a trap for oxides of nitrogen has to be capable of adsorbing the oxides of nitrogen at low temperatures up to the temperature necessary for the NOx reduction catalyst to function, the trap then allowing the oxides of nitrogen coming into contact with the DeNOx catalyst to desorb at a temperature sufficient to trigger the NOx reduction reaction.
Oxides with a crystallographic structure which generate channels with a size sufficient to accommodate oxides on nitrogen have been described (Japanese patent JP09075715A). The solids used in that Japanese patent are oxides containing Al and Sn or Zn and Sn mixtures, and have the crystallographic structure of hollandite.
French patent FR-A-2 733 924 describes a material with formula YBa2Cu3O7xe2x88x92x which can integrate the oxides of nitrogen in the mixed oxide composing the material. That patent indicates that the material, after being charged with oxides of nitrogen, is transformed by passing from an orthorhombic structure which is rich in oxygen to a tetragonal structure which is depleted in oxygen when the oxygen content of the gas reduces, and that phase transition causes desorption of oxides of nitrogen. According to that process, it is possible to influence adsorption and desorption of the oxides of nitrogen by varying the amounts of oxygen in the exhaust gases. It has recently been demonstrated (K-Y Lee, K. Watanabe, M. Misono, Applied Catalysis B 13, 241 (1997)) that the adsorption of NO in the presence of oxygen on the material YBa2Cu3O7xe2x88x92x leads to the formation of barium nitrate species (Ba(NO3)2). That same study also showed that that material suffers a dramatic loss of its oxides of nitrogen adsorption properties in the presence of carbon dioxide by forming barium carbonates. Since barium sulphate species are more stable than the nitrate species, it is feared that a compound of the YBa2Cu3O7xe2x88x92x type is also poisoned in the presence of sulphur dioxide by forming sulphate species on the oxides of nitrogen adsorption sites.
A. Clearfield and J. Stynes, J. Inorg. Nucl. Chem., 26 (1964), 117 and G. Alberti and E. Torracca, J. Inorg. Nucl. Chem., 30 (1968), 317 describe materials with a structure composed of phosphate tetrahedra and containing an element with an oxidation number of +4.
The invention concerns materials for adsorbing and desorbing oxides of nitrogen NO and NO2, in particular those present in exhaust gases, for example from the internal combustion engines of automotive vehicles functioning in a medium which is super-stoichiometric in oxidising agents, said materials being able to desorb NOx by raising the temperature. The materials are lamellar in structure and comprise phosphate tetrahedra and at least one element A selected from elements from groups IVB, VB, VIB, VIIB and IVA of the periodic table.
The adsorption and desorption material of the invention can trap oxides of nitrogen at low temperatures and desorb them at the temperature at which a deNOx catalyst is capable of reducing them. These materials are insensitive to the oxides of sulphur and carbon contained in the exhaust gases, which prevents the materials from being poisoned. The materials adsorb oxides of nitrogen over a wide temperature range while desorption is carried out in a very narrow temperature range which means that thermal regeneration is easy to control. During desorption, the oxides of nitrogen which have been adsorbed are released in bursts with a high NOx concentration, which is beneficial to the reaction kinetics for reduction of the desorbed oxides of nitrogen. The kinetics of the reduction of NOx by hydrocarbons are positive with respect to the oxides of nitrogen species. Said material does not have a basic oxide phase which substantially stabilises the oxides of nitrogen and oxides of sulphur into the nitrate and sulphate forms respectively. The SOx which can be inserted with the NOx into the structure of the material of the invention are desorbed in a temperature range which is similar to that of the NOx. Preventing the formation of stable sulphates ensures that poisoning of the adsorbing, material is minimal, meaning that the regeneration frequency and the regeneration temperature are lower, and thus the service life of the NOx trap is longer.
The present invention concerns materials for adsorbing and desorbing oxides of nitrogen, the structure of which is composed of phosphate tetrahedra containing at least one element (A) selected from elements from groups IVB, VB, VIB, VIIB and IVA of the periodic table. Preferably, this element (A) has an oxidation number of +4. The phosphate has a lamellar structure into which the NOx can insert at low temperatures and from which it can leave at high temperatures.
The adsorbing phase of the invention has a lamellar structure, it is composed of phosphate tetrahedra and it comprises:
at least one element (A) selected from the group formed by elements from groups IVB, VB, VIB, VIIB and IVA of the periodic table,
optionally, at least one element (B) selected from the group formed by alkali elements IA, alkaline-earth elements IIA, rare earth elements IIIB or transition metals from the periodic table,
optionally, at least one metal (C) selected from the group formed by precious metals from the platinum family (group VIII).
Preferably, the material of the invention contains at least one element (B) and preferably at least one element (C).
Elements (A) from groups IVB, VB, VIB, VIIB and IVA preferably have an oxidation number of +4 and are integrated into the octahedral sites arranged in the phosphate framework constituting the lamellae. Elements (A) are preferably selected from zirconium, titanium, germanium, or tin or a mixture of at least two of these elements. Preferably, zirconium or titanium are selected.
Elements (B) are selected from the group formed by the alkali elements IA, alkaline-earth elements IIA, rare earth elements IIIB or transition metals. They are located in the space between the lamellae of the lamellar material and at least partially compensate for the negative charge generated by the phosphate framework. An alkali metal is preferred, such as potassium or caesium.
Elements (C) are selected from the group formed by platinum, palladium, rhodium, ruthenium, iridium and osmium, preferably platinum. This implementation of the invention comprising at least one element (C) selected from noble metals from group VIII can oxidise NO to NO2. The structure of the materials used in the process of the invention is more effective for inserting the nitrogen dioxide (NO2) molecule than for inserting the nitric oxide (NO) molecule. Thus the presence of an element (C) can have a beneficial effect on the adsorption capacity of molecules of oxides of nitrogen contained in the exhaust gases.
The adsorbing phase of the invention has the following composition by weight, expressed as the percentage by weight with respect to the total mass of this active phase calcined at 1000xc2x0 C. for 4 h:
20% to 80% of phosphate, preferably 30% to 70% by weight, more preferably 30% to 60%, expressed as PO4;
5% to 40%, preferably 10% to 30%, of at least one element (A) selected from groups IVB, VB, VIB, VIIB, IVA of the periodic table;
optionally 1% to 55%, preferably 10% to 55%, of at least one element (B) selected from the group formed by alkali elements, alkaline-earth elements, rare earth elements or transition metals from the periodic table,
optionally, 0.05% to 5% of at least one metal (C) selected from the group formed by the precious metals from the platinum family.
The adsorbent phases can be in the form of a powder, beads, pellets or extrudates; they can also be deposited or directly prepared on monolithic supports of ceramic or metal. Advantageously, in order to increase the specific surface area of the materials and thus to increase their capacity to adsorb NOx, the materials can be deposited on large specific surface area porous supports such as silica or alumina (SiO2, Al2O3, TiO2, ZrO2, SiC, MgO . . . ) before being formed (extrusion, coating . . . ).
The process for preparing the material of the invention comprises the following steps:
(a) mixing an aqueous solution of a salt of element (A) with a concentrated phosphoric acid in the presence of a strong acid, or fluorocomplexes of element (A) are slowly decomposed in the presence of concentrated phosphoric acid and a strong acid, then heated under reflux to crystallise the complex,
(b) it is washed and dried to obtain an acidic compound which is lamellar;
(c) optionally, element (B) is introduced by dissolving the compound from step b) in a solution of a salt of element (B), to at least partially exchange the protons of the synthesised material with cations of at least one element (B) and then introduce element (B) in the cationic state into the lamellar structure of the compound,
(d) it is dried and calcined at at least 400xc2x0 C., element (B) remaining in the cationic state,
(e) optionally, at least one metal (C) is added using one of the following steps:
impregnating the compound from step d) with a solution of a precursor salt of at least one element followed by calcining or
prior to exchange with a cation of an element (B), proton exchange of the compound of step b), with a cation of at least one element (C), followed by calcining to form small aggregates of precious metal at the surface of the compound and to regenerate protons, the compound can then undergo a fresh cation exchange of protons by a cation of at least one element (B),
by intimate and mechanical mixing of the compound of step b) or d) with a powder composed of a support on which at least one element (C) is deposited,
f) optionally, pillars are formed in the lamellar structure using one of the following steps:
directly inserting one or more metallic complexes of elements from groups 13, 14 (IIIA, IVA) or transition metals into the lamellae by prolonged heating in an acid medium, or intercalating polar organic molecules (alkylamines, alcohols, amino acids) which are then exchanged with the metal complex or complexes;
calcining the inserted metallic species to form thermally stable oxide phases. The pillars formed are either clusters of the oxide of the metal or metals introduced, or aggregates composed of solid solutions of oxides of the metals introduced,
g) optionally, the compound is deposited or the compound is synthesised directly on a support with a large specific surface area, generally an inorganic refractory oxide.
The process for preparing materials used in the process of the present invention preferably comprises step f) and preferably comprises step g).
Step a) is carried out in an acidic medium at a pH below 5.
Step f) is carried out to increase the specific surface area of the material which is then in the range 30 to 500 m2/g, preferably in the range 100 to 500 m2/g, more preferably in the range 150 to 300 m2/g. Preferably, step f) is carried out after step b) and before step c). The weight of oxide pillars can represent up to 80% of the total dry weight of the adsorbing material.
The refractory inorganic oxide of step g) is generally selected from the group formed by the following compounds: alumina (alpha, beta, delta, gamma, khi, or theta alumina), silicas, silica-aluminas, zeolites, titanium oxide, zirconium oxide, divided carbides, for example silicon carbides, used alone or as a mixture. Mixed oxides or solid solutions comprising at least two of the above oxides can be added.
However, for use in a vehicle, it is usually preferable to use rigid supports (monoliths) with a large open porosity (more than 70%) to limit pressure drops which may cause high gas flow rates. and in particular high exhaust gas space velocities. These pressure drops are deleterious to proper functioning of the engine and contribute to reducing the efficiency of an internal combustion engine (gasoline or diesel). Further, the exhaust system is subjected to vibrations and to substantial mechanical and thermal shocks, so catalysts in the form of beads, pellets or extrudates run the risk of deterioration due to wear or fracturing.
Two techniques are used to prepare the catalysts of the invention on monolithic ceramic or metal supports (or substrates).
The first technique comprises direct deposition on the monolithic support, using a wash coating technique which is known to the skilled person, to coat the adsorbing phase prepared using the operating procedure described above (steps a), b), d), optionally c), e), f) and g)). The adsorbent phase can also be coated after the end of step c) and calcining step d) can be carried out on the phase deposited on the monolith.
The second technique comprises firstly depositing the inorganic oxide on the monolithic support then calcining the monolith between 500xc2x0 C. and 100xc2x0 C. so that the specific surface area of this oxide is in the range 20 to 150 m2/g, then coating the monolithic substrate covered with the inorganic oxide with the adsorbent phase obtained after steps b), c), d), e) or f) described above. Steps d) and e) can be carried out directly on the monolith after coating the gel obtained at c) on the monolithic substrate covered with refractory oxide. Preferably, step f) is carried out before the coating stage.
The monolithic supports which can be used are:
either ceramic, where the principal elements can be alumina, zirconia, cordierite, mullite, silica, alumino-silicates or a combination of several of these compounds,
or a silicon carbide and/or nitride;
or aluminium titanate;
or of metal, generally obtained from iron, chromium or aluminium alloys optionally doped with nickel, cobalt, cerium or yttrium. The most common are those known as FECRALLOY(copyright) or KANTHAL(copyright).
The structure of the ceramic supports is that of a honeycomb or they are in the form of a foam or fibres.
Metal supports can be produced by winding corrugated strips or by stacking corrugated sheets to constitute a honeycomb structure with straight or zigzag channels which may or may not communicate with each other. They can also be produced from metal fibres or wires which are interlocked, woven or braided.
With supports of metal comprising aluminium in their composition, it is recommended that they are pre-treated at high temperature (for example between 700xc2x0 C. and 110xc2x0 C.) to develop a micro-layer of refractory alumina on the surface. This superficial micro-layer, with a porosity and specific surface area which is higher than that of the original metal, encourages adhesion of the active phase and protects the remainder of the support against corrosion.
The quantity of adsorbing phase deposited or prepared directly on the ceramic or metallic support (or substrate) is generally in the range 20 to 300 g per liter of said support, advantageously in the range 50 to 200 g per liter.
The materials of the invention can thus adsorb and desorb oxides of nitrogen present in the gases, in particular exhaust gases.
These materials are characterized in that they are capable of adsorbing NOx at a temperature which is generally in the range 50xc2x0 C. to 400xc2x0 C., preferably in the range 100xc2x0 C. to 350xc2x0 C., more preferably in the range 150xc2x0 C. to 250xc2x0 C., said oxides of nitrogen being capable of being desorbed at a temperature generally in the range 300xc2x0 C. to 500xc2x0 C., preferably in the range 350xc2x0 C. to 450xc2x0 C. Thus the desorption of the invention can respectively take place within narrow ranges of temperature generally within a range of 80xc2x0 C. For light diesel cars, the temperature of the exhaust gas is generally in the range 150xc2x0 C. to 300xc2x0 C. and rarely exceeds 500xc2x0 C. The materials used in the process of the invention are thus suitable for adsorbing oxides of nitrogen present in the exhaust gases of stationary engines or, particularly, automotive diesel engines or spark ignition (lean burn) engines, but also in the gases from gas turbines operating with gas or liquid fuels. These gases are also characterized by oxides of nitrogen contents of a few tens to a few thousands of parts per million (ppm) and can contain comparable amounts of reducing compounds (CO, H2, hydrocarbons) and sulphur, also large quantities of oxygen (1 to close to 20% by volume) and steam. The material of the invention can be used with HSVs (volume of material per volume of gas per hour) of the exhaust gas generally in the range 500 to 150000 hxe2x88x921, for example in the range 5000 to 100000 hxe2x88x921.
The invention also concerns the use of materials for adsorbing and desorbing oxides of nitrogen in a process for eliminating oxides of nitrogen, more particularly in a medium which is super-stoichiometric in oxidising agents. Thus the material of the invention can be used in a process comprising:
an optional step for oxidising at least a portion of the oxides of nitrogen in the presence of an oxidation material,
a step for adsorbing at least a portion of said oxides of nitrogen onto an adsorption material as defined in the present invention,
a step for desorbing the oxides of nitrogen,
a step for selective reduction of at least a portion of the oxides of nitrogen to molecular nitrogen by reducing agents in the presence of at least one catalyst for reducing oxides of nitrogen.
Thus the process for eliminating oxides of nitrogen comprises, during the step for reducing the oxides of nitrogen, using a catalyst which is active and selective using reducing agents to reduce oxides of nitrogen to molecular nitrogen in a medium which is super-stoichiometric in oxidising agents. Catalysts for reducing oxides of nitrogen to nitrogen or nitrous oxide generally comprise at least one inorganic refractory oxide and can comprise at least one zeolite selected, for example, from MF1, NU-86, NU-87 and EU-1 zeolites and generally at least one element selected from elements from groups VIB, VIIB, VIII and IB, and transition metals. These catalysts can optionally contain at least one element selected from noble metals from group VIII, for example platinum, rhodium, ruthenium, iridium, palladium and optionally at least one element selected from elements from groups IIA the alkaline-earths and IIIB, the rare earths. Examples of catalysts for reducing oxides of nitrogen include the following combinations Cu-ZSM5, Fe-ZSM5, Ce-ZSM5, Pt-ZSM5.
The refractory inorganic oxide is selected from supports of the type Al2O3, SiO2, ZrO2 and TiO2, preferably alumina.
The reducing agents are selected from CO, H2, hydrocarbons, present in the fuel or added in the form of fresh products.
In the case where the material for adsorbing oxides of nitrogen of the present invention contains at least one element (C) selected from noble metals from groups 8, 9 and 10 of the periodic table, the process for eliminating oxides of nitrogen comprises a first step for oxidising and reducing oxides of nitrogen to nitrogen or nitrous oxide can take place directly on the adsorption material of the invention which permits both oxidation of oxides of nitrogen, trapping of the oxides of nitrogen, desorption of said oxides of nitrogen and reduction thereof.